A problem with defibrating apparatus of the type described is sealing the point where the shaft passes through the defibrator housing. Many solutions have been proposed, the most common one being to apply a packing material where the shaft passes through the housing, which material is pressed tightly against the drive shaft or against bearing surfaces on the shaft. These conventional seals have many drawbacks, however, the most important one being the substantail generation of heat, which may readily lead to overheating of the seal.